A property of fuel that is fed into a fuel tank (such as the cetane number of diesel fuel, the octane number of gasoline fuel, or concentration of alcohol fuel, for example) is not necessarily uniform and changed greatly depending on a country or a region. Thus, it can be said that the property of the fuel supplied to an internal combustion engine is not uniform either and that such a variation in the fuel property can be a cause to inhibit stabilizing a combustion state of the fuel.
Conventionally, it has been practiced to estimate the property of the fuel supplied to the internal combustion engine and to execute combustion control for combustion of the fuel on the basis of the estimated fuel property.
For example, Patent Document 1 describes an apparatus that estimates an alcohol fuel mixing rate of fuel supplied to an internal combustion engine on the basis of a detected signal from an air-fuel ratio sensor and that executes air-fuel ratio control on the basis of the estimated alcohol fuel mixing rate in the internal combustion engine for which either one of gasoline fuel and alcohol fuel or mixed fuel thereof can be used. In the apparatus described in Patent Document 1, when the fuel is fed, an estimated value that is estimated and stored before feeding of the fuel is adapted to be used for the air-fuel ratio control in a period until the entire fuel in a fuel pipe for supplying the fuel from a fuel tank to the internal combustion engine is consumed,